Recently, there has been an increasing interest in energy storage technology. Batteries have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention.
Among currently used secondary batteries, lithium secondary batteries developed in early 1990's have a higher drive voltage and a much higher energy density than those of conventional batteries using a liquid electrolyte solution such as Ni-MH batteries, Ni—Cd batteries, and sulphuric acid-lead batteries. For these reasons, the lithium secondary batteries are greatly spotlighted. Such a lithium secondary battery is assembled while interposing a separator between positive and negative electrodes. The separator is an important component that plays a role of ion passage in the battery and prevents the positive and negative electrodes from being directly contacted and thus short-circuiting the battery.
In the safety characteristics of the electrochemical device, overheating of the electrochemical device may cause thermal runaway, and explosion may occur when a separator is pierced. In particular, a polyolefin porous substrate commonly used as a separator of an electrochemical device shows extreme thermal shrinking behavior at a temperature of 100° C. or above due to the features of its material and its manufacturing process such as elongation, so there may occur an electric short circuit between positive and negative electrodes.
In order to solve the above safety-related problems of the electrochemical device, Korean Laid-open Patent Publication No. 10-2006-72065 and No. 10-2007-231 disclose a separator 10 having a porous coating layer formed by coating at least one surface of a porous substrate 1 having many pores with a mixture of inorganic particles 3 and a binder polymer 5 (see FIG. 1). In the separator 10, the inorganic particles 3 in the porous coating layer formed on the porous substrate 1 act as a kind of spacer that may keep a physical shape of the porous coating layer, so the inorganic particles 3 restrain thermal shrinkage of the porous substrate when the electrochemical device is overheated. In addition, interstitial volumes exist among the inorganic particles, thereby forming fine pores.
As mentioned above, the porous coating layer formed on the porous substrate contributes to the improvement of safety of the separator. However, the porous coating layer containing inorganic particles may deteriorate a united force between the electrode and the separator. Thus, during an assembling process of an electrochemical device such as a taking-up and assembling process of an electrode and a separator, the electrode and the separator may be more likely not closely adhered to each other but separated from each other, so lithium ions may be not effectively transferred, which may deteriorate the performance of a battery. In addition, the inorganic particles of the porous coating layer may be separated while the electrode and the separator are separated, and the separated inorganic particles may act as a local defect of the electrochemical device, thereby giving a bad influence on the safety of the electrochemical device.